Get modifier keys to show in Screencast when they are pressed by themselves
There is no setting which would allow displaying modifier keys alone. I looked into the source and the modifier keys are disabled in line 550 of the screencast addon:

ignore_keys = ['LEFT_SHIFT', 'RIGHT_SHIFT', 'LEFT_ALT',
         'RIGHT_ALT', 'LEFT_CTRL', 'RIGHT_CTRL', 'TIMER',
         'MOUSEMOVE', 'INBETWEEN_MOUSEMOVE']


You could right click on the screencast control to open the python source in the text editor and remove the keys want. Click register in the text editor's header and run the script. (Of course you could also save the modified version in into the blender installation folder).